This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. RNA interference (RNAi) is a broad-spread eukaryotic mechanism of gene silencing that plays a fundamental role in many aspects of human biology, including developmental timing, stem cell division, memory and learning. On the molecular level RNAi is mediated by a family of ribonucleo-protein complexes called RNA-Induced Silencing Complexes (RISC), which silence genes by mediating translational repression and degradation of targeted message RNAs (mRNA). The versatility and power of RNAi arises from the fact that RISC can be programmed to target any nucleic acid sequence for silencing. RISC programming is therefore a critical cellular function, requiring the action of a specialized macromolecular assembly called the RISC-loading complex (RLC). The goal of this proposal is to determine the 3D architecture of the human RLC. This information will offer mechanistic insight into the initiation of RNAi and may suggest novel strategies for the development of RNAi-based therapeutics.